An optical waveguide, such as an optical fiber is formed by a core section conveying the light signal, and a cladding section that surrounds the core to confine the light signal to the core. The light signal remains captive in the core by virtue of the difference between the refractive indexes of the core and the cladding sections and their geometries. In an optical fiber, the core section is cylindrical and the cladding surrounding it is tubular and in contact with the cylindrical core.
A fiber Bragg grating is an axial periodical change of the effective refractive index (n) that induce harmonic back reflection of the light component at a certain wavelength (λ), called the Bragg wavelength. The Bragg wavelength is related to the period length (Λ) of the effective index change by:λ=2nΛ  (1)
Variations of the Bragg wavelength due to the effects of the temperature (T) and strain (ε) can be calculated by the derivative of eq. 1:Δλ/λ=[(1/n)dn/dT+(1/Λ)dΛ/dT]ΔT+[1+(1/n)dn/dε]Δε  (2)Δλ/λ=(ζ+αf)ΔT+(1+pe)Δε  (3)where the thermo-optic coefficient for the fiber material, ζ=[(1/n)dn/dT; the coefficient of thermal expansion (CTE) of the fiber material, αf=(1/Λ) dΛ/dT; and the photo-elastic constant, pe=−(1/n) dn/dε.
This dependence of the Bragg wavelength on temperature and strain is sometimes useful in applications such as temperature and/or stress sensors. But for other applications, such as channel filtering of DWDM communication system, the Bragg wavelength must be kept constant for all environmental conditions. Consequently, various packaging have been developed in the past to maintain Bragg wavelength constant in changing environmental conditions. Active packages using energy to control the temperature and/or the strain of the Bragg grating are relatively expensive, require access to energy source and often need active monitoring of the Bragg wavelength reflection. To overcome this drawback, various passive package designs using the strain dependency of the Bragg wavelength to counter-balance its dependency to temperature have been proposed. These passive a thermal packages mostly use a mechanical structure to which the Bragg grating is fixed at a set tension and that would naturally impose to this Bragg grating a strain variation in function of the temperature according to the following equation:Δε/ΔT=−(ζ+αf)/(1+pe)  (4)Using eq. 4 to replace Δε in eq. 3, we obtain:Δλ/λ=(ζ+αf)ΔT+(1+pe)[−(ζ+αf)/(1+pe)]ΔT=0  (5)
The first category of those passive a thermal package designs is shown in FIG. 1 and it uses a combination of two materials with different coefficients of thermal expansion (CTE). The material having the lower CTE (αlow) serves as a substrate 50, while the material having the higher CTE (αhigh) serves as a thermal compensator 52. One end of the Bragg grating 54 is fixed to one end of the thermal compensator 52; the Bragg grating and the thermal compensator 52 are fixed in series to the substrate 50 at the other end. As the temperature increases, the thermal expansion of the thermal compensator 52 will push on the Bragg grating 54, decreasing its strain; inversely, as the temperature decreases, the thermal compensator 52 contracts and increases the strain in the Bragg grating. The strain variations in the Bragg grating can be expressed by the following equation:Δε/ΔT=(Lsubsαlow−Lcompαhigh)/(Lsubs−Lcomp)  (6)where Lsubs is the length of the substrate 50 and Lcomp is the length of the thermal compensator 52. Different athermal package designs using the serial thermal compensator mounting are described in the following US patents and US patent applications:
Patents orDate of grant/applications numberpublicationInventors5042898Aug. 27, 1991Morey et al.5844667Dec. 1, 1998Maron et al.5914972Jun. 22, 1999Siala et al.6112553Sep. 5, 2000Poignant et al.6374015Apr. 16, 2002Lin6377727Apr. 23, 2002Dariotis et al.6393181May 21, 2002Bulman et al.2002/0141700Oct. 3, 2002Lachance et al.2002/0146226Oct. 10, 2002Davis et al.2002/0150335Oct. 17, 2002Lachance et al.
This serial configuration arrangement has a number of drawbacks. Firstly, it increases the length of the device, which makes it unsuitable for applications where component footprint and density are important factors. Secondly, since the thermal compensator is located on only one side of the Bragg grating, when the device is placed in a thermal gradient environment, the Bragg grating and its thermal compensator will be at different temperatures inducing an offset in the resulting Bragg wavelength. This could be a major concern when these devices are used in photo-electronic modules where a lot of heat is generated locally at proximity of the devices.
Another popular configuration is the cantilever design, shown at FIG. 2. This approach also uses two materials with different CTEs, one as a substrate 60, the other for the thermal compensator 62. Both sides of the Bragg grating 64 are fixed at a set tension on top of the arms of a substrate 60 having an H shape. The thermal compensator 62 is fixed to the substrate arms parallel to the Bragg grating 64. If the thermal compensator 62 is placed in the upper arms section of the H shaped substrate 60, its CTE should be lower than the substrate; if it is placed in the lower section, its CTE should be higher than the substrate. Strain variations in the Bragg grating can be defined as follows:Δε/ΔT=HBragg(Lsubsαsubs−Lcompαcomp)/(HcompLBragg)   (7)where Hcomp is the parallel distance between the substrate 60 and the thermal compensator 62, and HBragg is the shortest of the parallel distance between the Bragg grating 64 and either the substrate 60 or the thermal compensator 62. Different athermal package designs using the cantilever thermal compensator mounting are described in U.S. Pat. Nos. 5,841,920, 6,044,189, 6,144,789, 6,175,674, 6,181,851, 6,295,399, 6,327,405, 6,370,310, 6,396,982 and 6,453,108. The cantilever configuration is also subject to thermal gradient offsets since the thermal compensator is located only at one side of the Bragg grating, but requires less footprint since it is parallel to the Bragg grating, instead of being in series with it. Also, by locating the thermal compensator close to the Bragg grating, the temperature gradient offsetting effect can by diminished, but not nullified.
Patents orDate of grant/applications numberpublicationInventors5841920Nov. 24, 1998Lemaire et al.6044189Mar. 28, 2000Miller6144789Nov. 7, 2000Engelberth et al.6175674Jan. 16, 2001Lin6181851Jan. 30, 2001Pan et al.6295399Sep. 25, 2001Engelberth6327405Dec. 4, 2001Leyva et al.6370310Apr. 9, 2002Jin et al.6396982May 28, 2002Lin6453108Sep. 17, 2002Sirkis
By using a substrate with negative thermal expansion, the need for a thermal compensator can be eliminated. Moreover, by fixing the fiber directly on that substrate reduces the parallel distance between the thermal compensation element and Bragg grating, reducing, but not nullifying, offsetting effects induced by a temperature gradient. That is the approach discussed in the following US patents and US patent applications numbers:
Patents orDate of grant/applications numberpublicationInventors5694503Dec. 2, 1997Fleming et al.6087280Jul. 11, 2000Beall et al.6187700Feb. 13, 2001Merkel6209352Apr. 3, 2001Beall et al.6233382May 15, 2001Olson et al.6240225May 29, 2001Prohaska6258743Jul. 10, 2001Fleming et al.6317528Nov. 13, 2001Gadkaree et al.6362118Mar. 26, 2002Beall et al.6377729Apr. 23, 2002Merkel6400884Jun. 4, 2002Matano et al.6403511Jun. 11, 2002Fleming et al.6477299Nov. 5, 2002Beall et al.6477309Nov. 5, 2002So6490394Dec. 3, 2002Beall et al.2001/0021292Sep. 13, 2001Merkel2001/0031692Oct. 18, 2001Fleming et al.2002/0146230Oct. 10, 2002So
Chemical composition and fabrication process of those substrates is critical to obtain the exactly matching negative CTE to compensate for thermal effects on the Bragg wavelength. In addition, those formulations must ensure repeatability, reproducibility and stability for all the operational conditions encountered by the devices. New formulations have to be developed for each change in fiber composition (chemical or geometrical) or Bragg grating exposure processes since these parameters will slightly change the Bragg wavelength dependency on temperature and strain. In this respect, U.S. Pat. No. 6,240,225 (Prohaska., May 29, 2001) proposes the use of an anisotropic negative substrate, where by changing the angle on which the Bragg grating is fixed to the substrate, the negative CTE can be adjusted for each type of fiber and grating.
In contrast to the serial and cantilever configurations discussed earlier which allow the possibility of fine tuning the Bragg wavelength after the fixing points are stabilized, the negative CTE substrate approach does not permit to readjust the Bragg. wavelength after curing and stabilization of the fiber anchoring points. Since those processes induce strain variation on the Bragg grating, they must be predictable and repeatable during the pre-tensioning for fiber fixation to insure acceptable yield.
U.S. Pat. No. 6,148,128 (Jin et al., Nov. 14, 2000) and U.S. Pat. No. 6,108,470 (Jin et al., Aug. 22, 2000) disclose yet a different athermal package design, using a negative thermal expansion substrate and a fine Bragg wavelength adjusting mechanism for post-assembly corrective tuning. These tuning mechanisms use programmable, latchable magnets to control gap distances between magnets due to magnetic force fields. In addition to increasing the cost of the package, the use of programmable, latchable magnets may cause some long-term reliability concerns resulting from changes of the magnetic properties of these magnets, as well as, changes of their equilibrium positions in the magnetic field as a result of mechanical shocks and vibrations. These patents, as well as U.S. Pat. No. 6,101,301 (Engelberth et al., Aug. 8, 2000) and U.S. Pat. No. 6,243,527 (Dawson-Elli, Jun. 5, 2001), also disclose another athermal configuration similar to the serial design, that is shown in FIG. 3. A thermal compensator 70 is parallel to the Bragg grating 72 and a low thermal expansion extension 74 is used to join the pushing end of the thermal compensator 70 to the pulled end of the Bragg grating 72. In this parallel configuration the thermal compensator 70 should have a higher CTE than the substrate 76; and since there is only one thermal compensator 70 on one side of the Bragg grating 72, it is sensitive to offsets due to a thermal gradient.
Tubular thermal compensator configurations covering the Bragg grating have also been proposed to avoid offsetting effects of temperature gradient. The most popular approach is to use a matching negative CTE coating material discussed in the following US patents/applications:
Patents orDate of grant/applications numberpublicationInventors4923278May 8, 1990Kashyap et al.6067392May 23, 2000Wakami et al.6233386May 15, 2001Pack et al.6466716Oct. 15, 2002Olge2002/0090174Jul. 11, 2002Girardon et al.
This approach presents the same drawbacks as the negative thermal expansion substrate, repeatability, reliability and absence of fine-tuning mechanisms. U.S. Pat. No. 6,449,293 (Pedersen et al., Sep. 10, 2002) and U.S. patent application Ser. No. 2002/01811908 (Pedersen et al., Dec. 5, 2002) propose to hold the Bragg grating between two negative matching thermal expansion substrates; an approach similar to negative CTE coating and which present the same major benefits and drawbacks. U.S. Pat. No. 6,147,341 (Lemaire et al., Nov. 14, 2000) discusses a tubular version of the cantilever configuration and U.S. Pat. No. 6,453,092 (Trentelman, Sep. 17, 2002) proposes a tubular version of the parallel configuration. U.S. Pat. No. 6,449,402 (Bettman et al., Sep. 10, 2002) also proposes a tubular version of the parallel configuration, as well as an axially symmetrical flat version of the parallel configuration. Both versions counteracts offsetting effects of thermal gradients without any mechanism for post-packaging fine-tuning, but the flat version is easier to assemble. Tubular versions usually require the Bragg grating to be threaded into the package, which limits the potential for automation of assembly, the length of the Bragg grating and makes difficult to produce multiple gratings in series along the same fiber. Another drawback of the tubular configuration is that since the grating is completely covered by the thermal compensator, it is impossible to deliver energy to it without affecting the thermal compensator; so, they are not compatible with grating writing on a pre-packaged fiber, nor to annealing or exposition tuning of the Bragg grating in a stabilized athermal package.
Another related art publication that is of interest to the present subject is U.S. Pat. No. 5,719,974 (Kashyap, Feb. 17, 1998) that proposes to cut open a window over an optical fiber laser diode's pigtail to enable laser writing of a Bragg grating on the fiber. U.S. Pat. No. 6,349,165 (Lock, Feb. 19, 2002) also proposes the use of two opposite windows cut on a tubular package to enable laser exposure of a pre-packaged fiber. The use of a matching negative CTE material to make the tube covering the Bragg grating, in which the two opposite windows are cut enables the athermalisation of the Bragg grating. Since the package is made of matching negative CTE material, it has the same drawbacks as all packages of that type, namely finding a formulation compatible with long term environmental stability and manufacturing repeatability. In addition, this package does not allow for post assembly tension fine tuning, nor for thermal compensation adjustment; and the tubular configuration requires fiber treading, which limits the automation potential and the use in serial athermal Bragg gratings applications. The energy beam must be strictly confined inside the windows apertures, otherwise the negative CTE material will absorb energy and contract, and so interfere with the grating formation; also, no method is provided to counteract the energy scattered by the fiber to reach and be absorbed by the negative CTE tube. U.S. Pat. No. 6,236,782 (Kewitsch et al., May 22, 2001) presents an athermal package for couplers comprising a Bragg grating in the waist region. The inert structure is designed to enable energy exposition of pre-packaged couplers, but no method is proposed to block scattering energy. The athermalisation is performed by a serially mounted thermal compensator, so it is subject to temperature gradient offsets. In addition, the requirement to tread the long structure is complex, rendering assembly automation difficult. Bragg wavelength and thermal compensation can be fine tuned after the grating is formed but not independently from one another.